1. Field of the Invention
The present invention relates generally to a handsfree communication apparatus intended to be connected to a wireless telephone set, and more particularly to a handsfree communication apparatus utilizing a plurality of microphones to enable a plurality of persons to simultaneously communicate with one another.
2. Description of the Related Art
A handsfree communication apparatus for a wireless telephone set conveniently allows communication without requiring the use of both of a user's hands, and mainly comprises a handsfree adapter, a microphone, and a loudspeaker. In the case of loading into a vehicle, the handsfree adapter is connected to the wireless telephone set lying within the vehicle, while the microphone and the loudspeaker associated with the handsfree adapter are firmly secured to predetermined points within the vehicle. This configuration will ensure that a person (such as a driver) within the vehicle is capable of transmitting speech by means of the microphone of the handsfree communication apparatus instead of a microphone of the wireless telephone set and receiving speech of the other party by use of the speaker of the handsfree communication apparatus in place of a loudspeaker of the wireless telephone set. In other words, this will enable the user to communicate with the other party without actually picking up the wireless telephone set.
Referring to FIG. 7, there is depicted, by way of example, a conventional handsfree communication apparatus. The handsfree communication apparatus generally designated at 100 is connected to a wireless telephone set 11. The apparatus 100 comprises a handsfree adapter 10, a microphone 12, and a loudspeaker 18. The handsfree adapter 10 includes an amplifier section 14 and a control section 16.
FIG. 8 depicts part of a circuit of the amplifier section 14 in which an indispensable or central constituent element is an operational amplifier 18. A signal (an voice signal) fed to an input terminal 20 is supplied via a resistor 22 to an inverting input terminal (-) of the operational amplifier 18. A constant-voltage power supply 24 is coupled to a non-inverting input terminal (+) of the operational amplifier 18. The thus amplified signal is provided through an output terminal 26. Reference numeral 28 denotes a feedback resistor.
In the configuration of FIG. 7, in the reception mode, a signal received by the wireless telephone set 11 is output through a receiving terminal 11a, subjected to a level regulation (a volume regulation) in the control section 16, and amplified in the amplifier section 14, and then sent to the loudspeaker 18 for speech conversion. In the transmission mode, an voice signal from the microphone 12 is amplified in the amplifier section 14, subjected to processing such as level regulation in the control section 16, and then fed to a transmitting terminal 11b. Then, a radio transmission is executed by the wireless telephone set 11.
However, the above-described conventional handsfree communication apparatus merely possesses a single microphone and a single loudspeaker, so that when two or more persons wished to take part in a simultaneous communication in the same vehicle, the plurality of persons had to speak into the single microphone 12. In addition, if the persons were sitting apart from each other, for example, in the front and back seats, the simultaneous communication was difficult to carry out.
On the contrary, the microphone receives not only speech but also noise (such as ambient noise, aliasing noise, and vehicular noise) having substantially the same level as that of the speech, whereupon the noise will be mixed into the voice signal, making the speech almost inaudible to the other party.
Japanese Patent Laid-open Pub. No. 114245/1989 discloses a handsfree communication apparatus equipped with a plurality of microphones, but is silent on noise cancellation. Japanese Utility Model Laid-open Pub. No. 21997/1990 discloses a handsfree communication apparatus intended to be mounted on a vehicle, in which a couple of microphones are accommodated in the same case, preventing persons sitting separately, for example, in the front and back seats, from taking part in simultaneous handsfree communication.